Don't worry-- the old "development charts" show that babies don't usually even START doing this UNTIL they're 4 months. If Savannah IS 4 months, then it's not like she's "behind". Plus, there's a window of time they do it in. If a chart says a 4 month old starts doing "such-n-such" then really, it's anywhere between 3 and 5 months. All babies progress differently, it's just that USUALLY, they accomplish a certain skill around a certain age.
Interestingly, a recent studies showed that over the past 20 years, infants have begun to develop slower anyway, and I wouldn't doubt if they make a change in the the milestone charts.
They cited several reasons for the "slow down":
#1 -- Daycare
As opposed to 20 years ago, 44% of infants are in
daycare. Because there's only 1 adult to anywhere from 4
to 6 babies depending on state regulations, babies in
daycare spend 90% of their day in a crib, swing, or baby
bouncer type of seat and therefore don't have the freedom
to move around and explore.
#2 -- Infant products
Overuse of products that keep baby "locked down".
Bouncy seats, swings,excersaucer type of seats, and
carseats that remove from the car and double as infant
carriers.
#3 -- Sleep and play position
The SIDS scare has parents putting babies to sleep on
their backs. They used to sleep on their tummies, and therefore developed "tummy skills" like pushing up on their arms, earlier than they do now. They still recommend that babies do not sleep on their tummies, but encourage parents to place babies on their tummies to play. (Alot of baby products always place babies on their backs... the toys that they're supposed to lie under, little blankets that have stuff hanging over them, etc.) Those are great-- but babies need tummy time too, and more than they USED to, since they're spending so much more time in carseats, cribs, swings, and now even in bed, they're on their backs.